The invention relates to a guard means for machines, more particularly pressbrakes, guillotines, stamping machines and the like in the case of which a first machine part performs working movements toward a second machine part, comprising a photoelectric detector arrangement positioned between the machine parts and attached to the moving machine part, a stop means adapted to halt working motion of the moving machine on interruption of a light beam and a deactivating means adapted to deactivate, at least directly prior to interruption of the light beam by the other machinepart, the stop means.
Such guard means are disclosed in connection with pressbrakes for example in the German patent publication 2,750,234, the European patent publication 0 264 347 B the European patent publication 0 146 460 A or the German patent publication 19,717,299 A. In the case of known guard means it is conventional for position monitoring valves to be employed to control the hydraulic drive of the moving machine part. Switching over the rapid working movement to the slower working motion below the predetermined threshold speed immediately before reaching the other machine part is effected by resetting the valves"" positions. Sensors monitor the valve position or other corresponding positions and then effect, on reaching the corresponding position, deactivation of the stop means or, respectively, of the photoelectric detector arrangement.
It is more especially in the case of older machines that upgrading modification of the machines themselves are necessary to mount the necessary sensors and to adjust same, this being a slow expensive process requiring some redesigning.
One object of the present invention is to so improve a guard means of the type initially mentioned that the deactivation of the stop means may be performed in a simpler fashion using devices, which may be readily mounted on existing plant without need for adjustment.
In accordance with the invention this object is attained because a speed measuring means for the moving machine part is provided and the deactivating means comprises means for at least partly deactivating the stop means below a predetermined threshold speed, a control means for the working movement of the moving machine part being adapted to reduce the speed immediately prior to interruption of the light beam by the other machine part to a speed below the threshold speed.
Owing to use of plain speed monitoring and speed control there is no need for later adjustment in position so that even for example in the case of modifications of the photoelectric detector arrangement no modification of the deactivating device will be necessary. A speed measuring means may be mounted in a simple manner externally on the machine or internally on the moving machine part. The possibilities for control variations are substantially more extensive in the case of the speed controlled guard means.
The dependent claims define features which represent advantageous developments and improvements of the guard means defined in claim 1.
It is advantageous for the deactivating means to possess means for deactivating the stop means when machine parts are moving apart, that is to say on the detection of negative speed values, since here there is no danger. In this respect the deactivating means may, in accordance with an advantageous form of the invention, be designed with means for storage of that position, at which the threshold speed is not reached during working movement and for deactivating the stop means until such position is reached again on the return motion of the moving machine part. This means that the deactivating means or, respectively, the photoelectric detector cannot be activated, when in the course of a return movement the moving machine part stops short of the stored position, something which otherwise would lead to activating the photoelectric detector arrangement and corresponding triggering of an undesired function.
In order to increase safety the speed measuring means comprises at least two independently operating measurement modules arranged at different positions, more particularly at two oppositely placed terminal portions of the moving machine part. This enhanced safety is more particularly produced by a comparison means for the data from the measurement modules and by means of the deactivating means for preventing deactivating of the stop means in the case of there being a no longer tolerable departure between data from the measurement module. Additionally it is possible in the case of discrepant data it is advantageously possible to prevent an entire operating movement of the moving machine part, something which also serves to increase safety.
In accordance with an advantageous design means are provided for the automatic detection of the overrun of the moving machine part following a stop signal at least in the case of the first operating movement, the deactivating means possessing means for preventing deactivation of the stop means on exceeding a predetermined maximum predetermined and allowable overrun distance. This measurement as well contributes to increasing safety and in this case as well additionally a complete working movement of the moving machine part may be prevented.
The means for automatically finding overrun preferably repeat such detection for each successive working movement until a permissible overrun distance has been detected. This means that operation of the machine with an excessive overrun distance is effectively prevented.
The speed measuring means preferably comprises at least one measurement module which possesses an incremental angular displacement sensor with a cable pulling means and/or an inductive sensor detecting the movement of a magnetic strip, a differentiating means being preferably provided for conversion of displacement signals into speed signals. Accordingly it is possible for position signals and speed signals to be produced by the same measurement module.